1. Field of the Invention
The present invention relates in general to photolithography. More particularly, it relates to a method for enhancing adhesion between a reworked photoresist and an underlying oxynitride film to avoid the reworked photoresist having line/space pattern collapse.
2. Description of the Related Art
As semiconductor manufacturers have sought to fabricate devices with a higher degree of circuit integration to improve device performance, it has become necessary to use photolithography with shorter wavelengths in the mid and deep UV spectra to achieve fine features. In the process of making the desired very fine patterns, many optical effects are experienced which lead to distortion or displacement of images in the photoresist that are directly responsible for wiring line width variations, opens, and shorts, all of which can lead to deteriorated device performance. Many of these optical effects are attributable to substrate geometry and reflectivity influences that include halation and other reflected light scattering effects which may occur due to uneven topography or the varying (wavelength dependent) reflectivity of the substrates and wires or layers being patterned thereon to define the desired features. Such effects are further exacerbated by both the non-uniformity of the photoresist film and film thickness. These effects are manifested in lithographic patterns; uneven line width, often with “reflective notching”, due to the standing wave effect, and non-vertical pattern sidewalls. Therefore, the application of an anti-reflect coating (ARC) layer has been developed to impede reflection of the light source and solve the standing wave phenomena.
In order to define very fine patterns, such as shallow trenches and contacts, the aspect ratio (AR) of the patterns, such as line/space patterns, in a photoresist layer is becoming higher.
However, critical dimension (CD) variations frequently occur in a photoresist pattern layer due to the high AR. Thus, a reworked photoresist layer is formed after the photoresist pattern layer is removed by ashing with oxygen-containing plasma. Because the plasma also reacts with the ARC layer, the surface structure of ARC layer is changed, reducing adhesion between the ARC layer and the subsequent reworked photoresist pattern layer.
FIG. 2 is a cross-section showing a collapsing photoresist pattern layer after rework of the prior art. In FIG. 2, a bottom anti-reflection coating (BARC) layer 101, such as silicon oxynitride (SiOxNy), is disposed on a semiconductor substrate 100. In general, when the CD of the photoresist pattern layer (not shown) formed overlying the BARC layer 101 is varied, the photoresist pattern layer is removed by, for example, ashing with oxygen-containing plasma for photoresist rework. Meanwhile, the BARC layer 101 is damaged by oxygen-containing plasma, changing the surface structure thereof. As a result, adhesion between the BARC layer 101 and the overlying reworked photoresist pattern layer 103 is reduced, causing the photoresist pattern layer 103 collapse, as shown in FIG. 2.